Charging material preheater with replaceable shaft wall portions

ABSTRACT

In a charging material preheater for preheating charging material ( 60 ) which is to be charged into a furnace vessel ( 3 ), having a shaft ( 9 ) which is fixed in a frame structure ( 20 ) and which in its upper region has a closeable feed opening ( 61 ) for the charging material ( 60 ) and a gas outlet and in its lower region a discharge opening for the charging material ( 60 ) and a gas inlet, and whose shaft walls ( 33, 35 ) delimit a receiving space ( 62 ) for the charging material ( 60 ) to be heated, the shaft walls ( 34, 35 ) are subdivided into shaft wall portions which are individually replaceably fixed in the frame structure ( 20 ).

TECHNICAL FIELD

The invention concerns a charging material preheater for preheatingcharging material which is to be charged into a furnace vessel. As setforth in the classifying portion of claim 1.

STATE OF THE ART

A charging material preheater of that kind is described for example inWO-A1-95/04910. An advantageous use of a charging material preheater ofthe general kind set forth is described in WO-A-90/10086. Here, an outersegment of the vessel cover of an arc furnace is replaced by a shaftwhich is fixed in a holding structure and through which the hot furnacegases can be passed. In heat-exchange relationship, they heat thecharging material which is disposed in the shaft, and make it possibleto achieve a substantial energy saving. The cross-section of thecharging material preheater in shaft form can be round or oval with asingle shaft wall. Preferably it is of quadrangular, that is to saypolygonal cross-section, so that the receiving space for the chargingmaterial to be heated is defined by four shaft walls.

The shaft wall or the shaft walls of known charging material preheatersare either formed from refractory material such as refractory bricks arefractory spray material or a refractory casting material or howeverfrom water-cooled steel wall elements, preferably in the form of tubularpanels.

If the shaft walls, on the inward side that is towards the internalspace of the charging material preheater, comprise refractory material.then, when a mechanical loading is involved as occurs when the shaft isloaded, that inward side is subjected to a greater degree of wear andthe risk of a higher level of damage than water-cooled steel wallelements. For that reason, and also for reasons of weight, operatorshave changed over to making the shaft walls in the form of fluid-cooledsteel walls, in particular in the form of tubular panels which can beconnected to a cooling circuit.

As already mentioned, the insides of the shaft walls, in the loadingoperation involving charging material, are exposed to high levels ofmechanical loading, in particular when heavy scrap is also used as thecharging material. If the heavy scrap contains for example railway railswhich have been cut and broken up into pieces, the sharp edges of suchrail pieces, upon being emptied into the shaft from a charging basketwhich is moved into a position above the upper loading opening of theshaft, break relatively large pieces out of the inside of the wall, inthe case of a shaft wall comprising,refractory material. Even in thecase of tubular panels which have a substantially higher level ofmechanical load-bearing capability, serious damage such as leaks canoccur due to such loadings.

Although basically the risk of damage to the inside of the shaft wallsis greater in the lower region of the shaft than in the upper region,because in the lower region the kinetic energy of the pieces of scrapwhich are falling from above into the shaft is greater, it is notspecifically possible to predetermine at what locations damage which hasto be repaired will occur in operation, so that it is also not possibleto obviate the need for local repair to the inside of the shaft wall byvirtue of precautionary strengthening at such locations.

If a repair becomes necessary, that involves relatively long stoppagetimes for the charging material preheater, particularly in the case ofrefractory walls, because of the necessary cooling-down time. Inaddition, eliminating leaks in the case of water-cooled walls requiresunacceptably long stoppage times because of the need to shut down thewater circuit and because of the necessary welding operations.

If fluid-cooled steel walls which can withstand a mechanically higherloading are used, that involves energy losses, in comparison with wallsin which the inside has a refractory cladding. For shaft cooling of amedium-size furnace, about 700 m³ of cooling water per hour is required.On average the cooling water is heated by 5° C. to 6° C. It can bededuced therefrom that, with 75 t of liquid steel which are produced in45 minutes, the average energy loss in the cooling water is about 3360kWh, that is to say about 45 kWh per tonne of liquid steel. In regard toa reduction in the level of energy losses, it would be desirable toreplace the more robust fluid-cooled steel walls by shaft walls ofrefractory material. The obstacle to that is a greater susceptibility torepair and longer stoppage times.

U.S. Pat. No. 3,632,094 A discloses a charging material preheatercomprising two shaft-form units which are of a similar structure andwhich can be fixed on both sides and symmetrically to a central portionwhich has a gas-permeable separating wall and which is rotatable through180°. To reduce the amount of wear and abrasion, that arrangement isintended to ensure that the charging material does not have to be movedfrom a charging opening present in the upper region through the entireshaft to a discharge opening in the lower region of the shaft, as in thecase of the charging material preheater as set forth in the classifyingportion of claim 1, and this arrangement provides that different kindsof charging material can be heated in two stages in the two portions,before the charging material is transported to the smelting vessel andcharged therein.

For operation of the known charging material preheater, it is necessaryfor the arrangement to have a central portion which is rotatable through180°, with a gas-permeable separating wall, and a device which afteremptying of the unit performs the rotary movement. Having regard to thehigh level of mechanical loading involved in the rotational procedure,the known charging material preheater is only suitable for preheatingrelatively small amounts of charging material.

STATEMENT OF THE INVENTION

The object of the invention is to reduce the repair and stoppage timesin a charging material preheater and therewith also tee costs in regardto local damage requiring repair at the inside of the shaft, inparticular in relation to shaft walls of refractory material. Theinvention further seeks to provide that the energy losses are reduced incomparison with a shaft comprising fluid-cooled steel walls.

That object is attained by the present invention. Advantageousconfigurations of the invention are to be found in the preferredembodiments of the invention.

In the charging material preheater according to the invention the shaftwall or, when the shaft is of a polygonal cross-section, the individualshaft walls, is or are subdivided into shaft wall portions which areindividually replaceably fixed in a frame structure. That makes ispossible on the one hand for the shaft, at the inside at individuallocations at which a lower level of mechanical loading is to beexpected, in particular in the upper region, to be lined with refractorymaterial or to be formed from refractory plates, while more robust steelwall elements are used at locations which are subjected to more, severeloadings. On the other hand, it permits damaged shaft wall portions tobe rapidly changed, irrespective of whether this involves a shaft wallportion of refractory material, that is in need of repair, or a leakingtubular panel. Preferably, in regard to the aspect of storage ofreplacement shaft wall portions, they are of such dimensions that thenumber of different sizes is minimized.

Shaft wall portions that have proven to be particularly advantageous arethose which, at their outside, have support surfaces with which theybear against corresponding co-operating counterpart mountings of theframe structure, when they are fitted into the frame structure. Theframe structure is preferably mounted in such a way that it can beraised and lowered in a holding arrangement, by means of which the shaftcan be pivoted or moved to the side from a position above a furnacevessel. If a damaged shaft wall portion has to be replaced, then theshaft is moved to the side out of the operating position above thefurnace vessel and the damaged shaft wall portion is replaced by a newor repaired one. After the shaft has been moved back again, the chargingmaterial preheater is again ready for operation.

Shaft wall portions comprising fluid-cooled steel wall elements are tobe capable of being individually taken out of the cooling circuit, tospeed up the replacement operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail by means of anembodiment with reference to nine Figures of drawings. The specificembodiment describes the use of the charging material preheater inaccordance with the invention in a modified arc furnace with a roundlower vessel portion in which the space conditions for the shaft whichis to be arranged beside the electrodes are limited, whereby theretaining members in the shaft are so designed that when the shaft isfilled, the wall at the electrode side is exposed to particularly highlevels of mechanical loading.

In the diagrammatic drawing:

FIG. 1 shows a smelting unit comprising an arc furnace and a chargingmaterial preheater which is disposed laterally beside the electrodes,with the vessel cover closed,

FIG. 2 shows the FIG. 1 unit with the shaft removed,

FIG. 3 shows a plan view of the smelting unit with the section III—IIIof the shaft, as shown in FIG. 2.

FIG. 4 shows the section IV—IV in FIG. 3 with the shaft arranged overthe furnace vessel,

FIG. 5 shows a part on an enlarged scale of FIG. 4,

FIG. 6 shows a part on an enlarge scale of FIG. 5 illustrating thefixing of the shaft wall portions in the frame structure,

FIG. 7 is a view corresponding to FIG. 6 of a modified shaft wallportion, and

FIG. 8 and FIG. 9 are each partly sectional, highly diagrammaticperspective views of the parts, which are relevant to understand thearrangement, of the shaft disposed above a furnace vessel.

DETAILED DESCRIPTION OF THE DRAWINGS

The smelting unit illustrated in FIGS. 1 to 5 includes an arc furnace 1with a furnace vessel 3 mounted on a furnace cradle 2, and a vesselcover 4 of an arch-like shape which covers the upper edge of the furnacevessel. The furnace vessel 3 comprises a lower vessel portion 5 whichforms the brick-lined furnace hearth for receiving the molten metal, andan upper vessel portion 6 which is usually formed from water-cooledelements. As can be seen in particular from FIGS. 3 to 5, the vesselcover 4 comprises a first cover portion 7 which is shown in theoutwardly pivoted position in FIG. 3, and a second cover portion 8 whichis substantially formed by the lower end portion of a shaft 9 or a frame10 accommodating the lower portion of the shaft 9 (FIGS. 1 and 2). InFIG. 1 the two-part vessel cover is closed, in FIG. 2 the second coverportion including the shaft 9 is extended.

As shown in particular by FIGS. 3 to 5, the part shown in the drawingsto the right of the center of the furnace vessel corresponds to aconventional arc furnace with a round furnace vessel and electrodes 12which can be moved into the furnace vessel concentrically with respectto the vessel center 11 (central axis of the vessel, see FIGS. 3 and 4).Only the region illustrated in the drawings to the left of theelectrodes 12 is modified above the lower vessel portion, in comparisonwith the usual configuration of an arc furnace with a round vesselshape.

The first cover portion 7 is of an arch-like configuration and has aso-called cover heart or core portion 13 with electrode openings 14(FIG. 5) for three electrodes 12 which are to be introduced into thevessel, in the usual triangular arrangement of a three-phase arcfurnace. The electrodes 12 are mounted to electrode carrier arms 15 andcan be raised/lowered and pivoted to the side by means of an electrodelifting and pivoting arrangement 16. The first cover portion 7 can belifted by means of a cover lifting and pivoting arrangement 17 from theposition shown in FIGS. 4 and 5 in which it lies on the edge of thevessel, and can be pivoted to the side into the position shown in FIG. 3in order to open the furnace vessel for example for a basket chargingoperation from above. A suitable cover lifting and pivoting arrangementis described for example in EP-0 203 339.

In the illustrated embodiment not only the furnace vessel 3 but also thecover lifting and pivoting arrangement 17 and the electrode lifting andpivoting arrangement 16 are fitted onto the furnace cradle 2 so that thefurnace vessel can be tilted jointly with the electrodes.

So that the electrode arrangement does not have to be changed in aconversion or modification operation, the construction illustratedprovides that the first cover portion is in the form of an oval 19 whichis defined by a chord 18 and which includes the usual electrodeconfiguration. When the first cover portion is fitted on the furnacevessel, the chord must lie in the tilting direction, that is to sayperpendicularly to the plane of the paper in the view shown in FIG. 2.In that way, the furnace vessel can be tilted for the tapping-offoperation or the slag removal operation, with the cover portion 7 closedand without displacement of the cover portion 8. In that situation, theshaft 9 only has to be slightly lifted. In that way the heat losses dueto radiation are reduced or the hot furnace gases pass for the majorpart into the preheating shaft. Possibly, the gap which occurs when theshaft 9 is raised between the lower edge of the shaft or the secondcover portion 8 and the vessel edge (39 in FIG. 5) can be sealed off bymeans of an apron or other means which are mounted to the shaft or tothe vessel edge.

The shaft 9 is fixed in a frame structure 20 which encloses the shaft 9in the manner of a cage, the frame 10 illustrated in FIGS. 1 and 2 ofthe second cover portion 8 representing a part of the frame structure.The frame structure 20 which is illustrated in greater detail in FIGS. 5to 9 and which carries the shaft 9 is mounted in a holding arrangement21 (see FIGS. 1 to 3) in such a way that the frame structure 20 can beraised and lowered jointly with the shaft by means of a liftingarrangement 22. For that purpose, provided on transverse beam members 23of the frame structure are engagement locations 24 for the liftarrangements 22 which are supported on the holding arrangement 21 sothat the, transverse beam member 23 and therewith the frame structure 20carrying the shaft can be lifted out of the lower position shown in FIG.1 into the upper position shown in FIG. 2. In that case, the requiredguidance effect is ensured by guide bars 25.

The holding arrangement 21 with shaft 9 is horizontally displaceable.For that purpose, rails 27 are provided on a support structure 26 andthe holding arrangement 21 is provided with wheels 28 which permitdisplacement of the holding arrangement 21 in the horizontal direction.

The shaft 9 can be closed at the top by means of a shaft cover 29 which,in the illustrated embodiment, is displaceable horizontally on rails inorder to open the upper charging opening 61 for charging by means of acharging basket 31 (FIG. 4) transported by a crane. On the side which isthe rear side in FIG. 1, the shaft cover 29 which is of a cap-like ordome-like configuration has a gas passage opening 32 connected to awaste gas conduit 33 when the shaft 9 and therewith the frame 10 are inthe position shown in FIG. 1.

FIGS. 3, 8 and 9 show that the shaft 9 is of rectangular cross-sectionalconfiguration. Preferably, the shaft is rectangular in the lower regionwhen the assembly has retaining members for the feed or chargingmaterial, as are described in greater detail hereinafter. Therefore theshaft has shaft walls which are arranged in a rectangular shape at leastin its lower region, with a front shaft wall 34 which is adjacent to thechord 18 of the first cover portion 7 when the vessel cover is closed(FIGS. 1, 4 and 5), a rear shaft wall 35 which is remote from the chord18, and two lateral shaft walls 36 and 37 which connect those walls. Inthat case, the front shaft wall 34 is of approximately the same lengthas the chord 18, that is to say the shaft wall 34 adjoins the chord 18,with a narrow cover gap 38. The cover gap is shown on an enlarged scalein FIG. 5.

It should be noted at this point that, in the case of a dome-like vesselcover, as is shown in FIGS. 4 and 5, the chord is only a straight linein plan view but otherwise it is a line following the section of thedome profile, and thus also the lower edge of the front shaft wall 34 isof the same shape.

When the vessel cover is closed, that is to say in the condition shownin FIGS. 1, 4 and 5, the external contour of the vessel cover is formedfrom the lower edge of the rear shaft wall 35, the lower edge of the twoadjoining lateral shaft walls 36 and 37, and the oval part 19, adjoiningsame, of the first cover portion 7. The upper vessel edge 39, that is tosay the upper edge of the upper vessel portion 6, is adapted to thatcontour. The contour of the upper edge 39 of the vessel thus correspondsto an oval defined by a trapezoidal line (wall sectors 40 a, 41 a).

The transition from the sectors of the edge of the vessel, which aredefined by the trapezoidal line, to the respective sector of the roundcross-section of the lower vessel portion is made by a converging wallsector 42 a of the upper vessel portion 6 (see FIG. 3).

As already mentioned and as shown in FIG. 5 the first cover portion 7 isseparated from the second cover portion 8 by a gap 38 which extendsparallel to the chord 18 so that the furnace vessel can be tilted in thedirection determined by the furnace cradle, in which there are arrangeda tapping hole 43 and a working opening 44, as viewed from the center 11of the vessel, without that being impeded by the adjoining front wall 34of the shaft 9. As the second cover portion 8 and therewith the shaft 9is fixed in a holding arrangement carried by the support structure 26,that is to say not on the furnace cradle, that part of the cover cannotalso tilt. It is sufficient however for the lower edge of the shaft tobe slightly lifted away from the upper edge 39 of the vessel in order topermit slight tilting movements of the furnace vessel with the firstcover portion lying thereon and with the electrodes inserted.

In order to prevent furnace gases from escaping through the gap 38between the two cover portions, means for sealing off the cover gap 38are provided at least one of the mutually adjoining edges 45 and 46respectively of the first and second cover portions.

In the illustrated embodiment, a sealing gas 47 is blown into the gap 38for sealing purposes. For that purpose, provided along the edge 46, thatis to say at the front shaft wall 34, is a duct 13 with a slit-shapednozzle opening which is towards the cover gap 38, or a row of holes.

In addition, provided at the edge 45 of the first cover portion is astrip arrangement 51 which is formed by cooling tubes and which, whenthe cover is closed, engages with clearance into a groove.

Preferably the shaft 9 is provided with retaining members 54 (fingers)for the charging material. The kind of retaining members described in WO95/04910 is particularly suitable for that purpose. Depending on therespective contour of the upper vessel edge 39, 40, 41 and theconfiguration of the converging wall sector 42 however those retainingmembers 54 need to be of a special configuration and arrangement.

In the illustrated embodiment, the transition from a rectangularcross-section of the shaft 9 to the round cross-section of the lowervessel portion 5 is formed by way of a polygonal cross-section which inthis example follows a trapezoidal line. The transition is already begunabove the upper edge 39 of the upper vessel portion insofar as, in thelower shaft portion which is below the retaining members 54, the cornersbetween the shaft walls 35, 36 and 35, 37 are of a configuration such asto converge towards the center of the vessel. The converging shaft wallsectors are identified by references 58 and 59 (see FIG. 3). There areflat surfaces which convert the rectangular cross-section into across-sectional profile of the walls 36, 35 and 37, which follows atrapezoidal line and which is then reflected in the profile of the uppervessel edge 39 by straight portions 40 a and 41 a. The furthertransition from the contour of the upper vessel edge 39, which in theregion below the shaft 9 follows a trapezoidal line, to the roundcross-section of the lower vessel portion, is by means of a convergingwall sector 42 a.

The pivotable fingers 54 are arranged parallel and at a spacing fromeach other (see FIG. 3) and are mounted in rotary mountings 56 arrangedin the frame structure 20 at the rear shaft wall 35. The pivotablefingers 54 are pivotable downwardly from the closed position which isshown in FIG. 5 in solid line and in which the inner portions of thefingers project into the internal space of the shaft and preventcharging material from passing therethrough, into a release positionwhich is shown in dash-dotted line in FIG. 5 and in which the innerportions of the fingers point downwardly and permit the chargingmaterial to pass through the shaft. The pivotable fingers 54 are alsoinclined obliquely downwardly at an angle of about 20° relative to thehorizontal, in the closed position.

The fingers 54 which are disposed above the shaft wall sectors 58 and 59cannot be pivoted downwardly as far as the middle fingers, FIG. 3 showsthe release position of the fingers 54 in solid lines, with the closedposition in broken lines. It will be seen that three, respective fingerswhich are adjacent to the shaft walls 36 and 37 and for which themaximum open position is illustrated cannot be pivoted downwardly as faras the central fingers. That presupposes individual actuation of thepivotal movement of those fingers while the central fingers can bepivoted jointly.

Upon the joint downward pivotal movement of the pivotable fingers 54from the closed position into the release position, the chargingmaterial is guided by the pivotable fingers 54 towards the center, thatis to say into the round lower vessel portion, so that the wall sector42 of the upper vessel portion is protected from an excessively greatloading.

FIGS. 6 to 9, beside FIG. 5, show further details of the structure andarrangement of the shaft 9.

As is shown in particular by FIGS. 8 and 9 which for the sake ofimproved understanding reproduce greatly simplified diagrammaticillustrations of the invention, the shaft 9 is fixed in a framestructure 20 which surrounds the shaft in a cage-like configuration. Theframe structure 20 includes at the corners four perpendicular tubes 63,64, 65 and 66 which are welded by horizontal tubes 67, 68 and 69 in alower, central and upper plane to afford a cage-shaped frame structure20. The shaft walls 34 to 37 are respectively subdivided into lowershaft wall portions identified by /1 and upper shaft wall portionsidentified by /2, of which only individual ones are illustrated in theFIGS. FIG. 8 for example only shows the shaft wall portions 34/1, 35/2,37/1 and 37/2. In that respect moreover the rear shaft wall portion 35/2is further subdivided in the horizontal direction into three portions ofequal width, of which FIGS. 8 and 9 only show the respective shaft wallportion 35/2 adjoining the wall 37. The lower shaft wall portions 34/1to 37/1 which in this application are identified as first shaft wallportions are designed as fluid-cooled steel wall elements in the form oftubular panels 70. The tubular panels 70 have tube portions which aredisposed in mutually juxtaposed relationship and which are connectedtogether at the ends and which extend perpendicularly, that is to say inthe direction of movement of the charging material upon filling andemptying of the shaft.

The upper shaft wall portions 34/2 to 37/2 are in the form of refractorywall elements. On the inside which is towards the receiving space 62 forthe charging material 60, they have a layer or plate 71 consisting ofrefractory material. While the second wall portions 34/2 shown in FIGS.5 and 6 are formed by refractory wall elements 71 which have afluid-cooled outside in the form of a cooling box 72, the second shaftwall portion 34/2 illustrated in FIG. 7 is constructed without fluidcooling. The refractory material is introduced into a sheet metal platemember 73 with edges which are angled towards the shaft interior.

In order to be able to change the shaft wall portions quickly, at ,theiroutside they have angular support surfaces 74 which are mounted onsupport projections 75 and which are supported on angular co-operatingcounterpart mountings 76 which are welded to suitable locations of thehorizontal tubes 67, 68 and 69 respectively of the frame structure 20.Corresponding, easily releasable fixings which permit fast individualreplacement are provided both in relation to the first and also thesecond wall portions.

In the event of damage to a wall portion, it can be quickly replaced byanother wall portion of the same kind, after the shaft has been emptiedand moved out. It is desirable for the wall elements, as far aspossible, to be standardized in regard to their size and the releasablefixing means thereof in the frame structure, so as to be able to reducethe storage thereof to a minimum. It should be possible for the coolingcircuits to be individually taken out of operation.

In regard to the reduction in energy losses it has proven to beadvantageous to use cast steel plates at locations subjected to a highlevel of mechanical loading: the temperature of the cast steel plates,which is detected by at least one temperature sensor, is kept just belowthe temperature at which deformation of the cast steel plate occursunder the mechanical loading in the charging operation. That temperatureis about 800° to 1000° C.

We claim:
 1. A charging material preheater for preheating chargingmaterial (60) which is to be charged into a furnace vessel (3) having ashaft (9) which is fixed in a frame structure (20) and which in itsupper region has a closable feed opening (61) for the changing material(60) and a gas outlet (32) and in its lower region a discharge openingfor the charging material (60) and a gas inlet, and shaft walls (32 to37) that delimit a receiving space (62) for the charging material (60)to be heated, characterized in that at least one of the shaft walls (34to 37) is subdivided into shaft wall portions (34/1, 34/2 to 37/1, 37/2)which are individually fixed in the frame structure (20) and areindividually replaceable.
 2. A charging material preheater according toclaim 1 characterized in that the shaft walls (32 to 37) define a shaftaxis and in that the at least one shaft wall (34, 35) is subdivided in adirection perpendicular to the shaft (9) axis into at least two shaftwall portions (34/1, 34/2; 35/1, 35/2) which are arranged one above theother.
 3. A charging material preheater according to claim 1characterized in that the at least one shaft wall (34, 35) includes atleast one shaft wall portion (34/1, 35/1) which is in the form of asteel wall element (70) which can be cooled by a forced coolingapparatus.
 4. A charging material preheater according to claim 3characterized in thatthe steel wall element is in the form of afluid-cooled tubular panel.
 5. A charging material preheater accordingto claim 4 characterized in that the tubular panel is formed by tubeportions which are disposed in mutually juxtaposed relationship andwhich extended parallel to the direction of movement of the chargingmaterial upon filling and emptying of the shaft.
 6. A charging materialpreheater according to claim 3 characterized in that the steel wallelement is in the form of a cast steel plate and the temperature of thecast steel plate is controllable by the forced cooling apparatus independence on a temperature measurement value of the cast steel plate,which value can be detected by at least one temperature sensor.
 7. Acharging material preheater according to claim 1 characterized in thatthe at least one shaft wall (34, 35) includes at least one, second shaftWall portion (34/2, 3512) which is in the form of a refractory wallelement and which, on the inside which is towards the receiving space(62) for the charging material (60), is formed by a layer or plate (71)of refractory material.
 8. A charging material preheater according toclaim 7 characterized in that the refractory wall element has afluid-cooled outside (72).
 9. A charging material preheater according toclaim 7 characterized in that the refractory wall element is formed by asheet metal plate member (73) with edges which are angled towards tieshaft interior and into which is fitted a refractory lining (71).
 10. Acharging material preheater according to claim 3 characterized in thatthe at least one shaft wall (34, 35) comprises fluid-cooled shaft wallportions which are individually connectable to cooling circuits of theforced cooling apparatus, so that the fluid-cooled shaft wall portionscan individually be taken out of operation.
 11. A charging materialpreheater according to claim 1 characterized in that the shaft wallportions (34/1, 3412; 35/1, 35/2) have at their outside support surfaces(74) which are supported on cooperating counterpart mountings (76) ofthe frame structure (20).
 12. A charging material preheater according toclaim 1 characterized in that first shaft wall portions (34/1 to 37/1)are provided in the lower region of the shaft (9) and second shaft wallportions (34/2 to 37/2) are provided in the upper region of the shaft.13. A charging material preheater according to claim 1 characterized inthat the frame structure (20) is in the form of a cage structure (63 to69) which surrounds the shaft walls (34 to 37).
 14. A charging materialpreheater according to claim 3, wherein the shaft (9) has in its lowerregion retaining members (54, 55) for the charging material to beheated, which are formed by fingers (54) which are arranged parallel andat a spacing from each other and which are mounted in rotary mountings(56) and which are pivotable downwardly from a closed position in whichinner portions with inner ends of the fingers (54) project inclinedlydownwardly into the interior of the shaft to block the passage ofcharging material therethrough, into a release position in which saidportions of the fingers (54) point downwardly and release the chargingmaterial to pass trough, characterized in that the shaft wall (34) whichis located opposite to the inner ends of the fingers, when the fingersproject into the interior of the shaft to block the passage of chargingmaterial, includes one or more first shaft wall portions in the regionwhich is subjected to a particular mechanical loading due to theresilient return movement of the fingers en the shaft is loaded.
 15. Asmelting unit having an are furnace (1) which includes the furnacevessel (3) with a vessel cover (4) comprising fist and second coverportions (7, 8) of which the first cover portion (7) includes at leastone electrode opening (14) for the introduction of an electrode (12) andthe second cover portion (8) includes the shaft (9) which is fixed in aholding arrangement (21), characterized in that the shaft (9) isconstructed according to claim 1 and the holding arrangement (21) andthe vessel (3) are displaceable horizontally relative to each other. 16.A smelting unit according to claim 15 characterized in that the framestructure (20) carrying the shaft (9) in such a way that it can beraised and lowered in the holding arrangement (21).
 17. A chargingmaterial preheater according to claim 1 characterized in that thecharging material which is to be charged into the furnace vessel ismetal scrap.